The present invention relates broadly to thermal management devices for electronic components, such as integrated circuit (IC) chips. More particularly, the invention relates to a thermal dissipator, i.e., heat sink, for attachment to the heat transfer surface of an electronic component for the conductive and/or convective cooling of the component.
Circuit designs for modem electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like have become increasingly complex. For example, integrated circuits have been manufactured for these and other devices which contain the equivalent of hundreds of thousands of transistors. Although the complexity of the designs has increased, the size of the devices has continued to shrink with improvements in the ability to manufacture smaller electronic components and to pack more of these components in an ever smaller area.
As electronic components such as IC semiconductor chips or dies have become smaller and, in turn, more densely packed on printed circuit boards (PCBs), designers and manufacturers now are faced with the challenge of how to dissipate the heat which is ohmicly or otherwise generated by these components. Indeed, it is well known that many electronic components, and especially power semiconductor components such as transistors and microprocessors, are more prone to failure or malfunction at high temperatures. Thus, the ability to dissipate heat often is a limiting factor on the performance of the component.
In basic construction, and as is described in U.S. Pat. Nos. 5,488,254 and 5,359,768, semiconductor chips or dies typically are packaged by encapsulation in a ceramic or plastic chip carrier. External connections provided on the chip carrier allow for the chip to be mounted onto a PCB by wire bonding electrical leads on the carrier through a common mounting surface on the board, or by surface mounting the carrier directly to the mounting surface of the board. Recently, the industry trend has been away from ceramic chip carrier packages and toward plastic packages. Usually molded of an engineering thermoplastic material such as polyethylene terephthalate (PETP), polyphenylene sulfide (PPS), polyetherimide (PEI), polyetherether ketone (PEEK), polyetherketone (PEK), or polyimide (PI), or a thermosetting material such as an epoxy or an epoxy-phenolic composite, these plastic chip packages typically are less expensive than their ceramic counterparts. However, these plastic materials generally exhibit less efficient heat transfer characteristics as compare to other package materials, and therefore may raise additional thermal dissipation considerations.
Electronic components within integrated circuits traditionally have been cooled via forced or convective circulation of air within the housing of the device. In this regard, cooling fins have been provided as an integral part of the component package or as separately attached thereto for increasing the surface area of the package exposed to convectively-developed air currents. Electric fans additionally have been employed to increase the volume of air which is circulated within the housing. For high power circuits and the smaller but more densely packed circuits typical of current electronic designs, however, simple air circulation often has been found to be insufficient to adequately cool the circuit components. One approached has been to design integral metal or ceramic heat sinks into the die package or mounting assembly, such as is shown, for example, in U.S. Pat. Nos. 5,175,612; 5,608,267; 5,605,863; 5,525,835; 5,560,423; and 5,596,231.
Heat dissipation beyond that which is attainable by simple air circulation may be effected by the direct mounting of the electronic component to a thermal dissipation member such as a “cold plate” or other heat sink. The heat sink may be a dedicated, thermally-conductive metal plate, or simply the chassis or circuit board of the device. To improve the heat transfer efficiency through the interface, a layer of a thermally-conductive interface material typically is interposed between the heat sink and electronic component to fill in any surface irregularities and eliminate air pockets. Initially employed for this purpose were materials such as silicone grease or wax filled with a thermally-conductive filler such as aluminum oxide, magnesium oxide, zinc oxide, boron nitride, and aluminum nitride. Such thermal interface materials usually are semi-liquid or solid at normal room temperature, but may liquefy or soften at elevated temperatures to flow and better conform to the irregularities of the interface surfaces.
Alternatively, another approach is to substitute a cured, sheet-like material in place of the silicone grease or wax. Thermal interface materials of this type may be compounded as containing one or more thermally-conductive particulate fillers dispersed within a polymeric binder, and may be provided in the form of cured sheets, tapes, pads, or films. Typical binder materials include silicones, urethanes, thermoplastic rubbers, and other elastomers, with typical fillers including aluminum oxide, magnesium oxide, zinc oxide, boron nitride, and aluminum nitride. Materials of this type are further described in U.S. Pat. Nos. 6,096,414; 5,545,473; 5,533,256; 5,510,174; 5,471,027; 5,298,791; 5,213,868; 5,194,480; 5,137,959; 5,060,114; 4,979,074; 4,974,119; 4,869,954; 4,654,754; and 4,606,962, and in WO9637915.
More recently, thermal interface materials of a phase-change type, more commonly known as phase-change materials (“PCM's”), have been introduced which are self-supporting and form-stable at room temperature for ease of handling, but which liquefy or otherwise soften at temperatures within the operating temperature range of the electronic component to form a viscous, thixotropic phase which better conforms to the interface surfaces. These PCM's, which may be supplied as free-standing films, or as heated screen printed onto a substrate surface, advantageously function much like greases and waxes in conformably flowing within the operating temperature of the component under relatively low clamping pressures of about 5 psi (35 kPa). Such materials are further described, for example, in U.S. Pat. Nos. 6,054,198 and 6,523,608, and in US20030203188; US20030152764; US20020135984; WO0036893; and WO02059965.
Yet another approach for the cooling of electronic components, and particularly components that are densely packed on a circuit board, involves the use of a metal foil thermal dissipator. As is detailed in commonly-assigned U.S. Pat. No. 5,550,326, such dissipator includes a light-weight, thermal dissipation layer formed of a relatively thin, e.g., 1–30 mil, and flexible copper or other metal foil sheet, and an attached pressure-sensitive adhesive pad for bonding the foil sheet to a surface of the electronic component. As compared to more conventional cast or extruded metal plate, fin, pin, or other heat sinks, such as those shown in U.S. Pat. Nos. 6,650,215; 6,269,002; 5,486,980; 5,381,859; 5,304,846; 5,294,831; 5,241,452; 5,107,330; 5,049,981; 4,953,634; 4,765,397; and 4,703,339, such dissipator is lighter and less expensive, has a lower profile to accommodate different mounting opportunities even in a relatively confined spaces, eliminates the need for a clip or other mechanical attachment means, and is readily removable for repair or replacement of the component. Dissipators of such type are marketed commercially under the name T-Wing™, by the Chomerics Division of Parker-Hannifin Corp., Woburn, Mass., as including a 7 mil (0.175 mm) thick sheet of copper foil which is laminated on both sides with an electrically-insulating polymeric film laminated on both sides. A 2–3 mil (0.051 mm) thick silicone pressure sensitive adhesive pad is affixed to on side of the foil sheet for the attachment of the dissipator to the surface of the die package. A variation of the above-described metal foil dissipator, as further described in commonly-assigned U.S. Pat. No. 6,705,388, uses a thin ceramic tile instead of the foil.
In view of proliferation of electronic devices, it will be appreciated that improvements in the design of thermal dissipators therefor would be well-received. Especially desired would be a heat sink which is efficient, yet inexpensive and easy to use.
The present invention is directed to a heat sink construction which is not only low in cost and light in weight, but which is also efficient as well as conformable to accommodate both micro irregularities and macro curvatures and other deviations in planarity in the electronic package or other surface to which the heat sink may be attached.
In an illustrative embodiment, the heat sink includes a base portion for attachment to the electronic package or other surface, and a cellular, radiator-like body portion which is bonded or otherwise joined to a surface of the base portion. The base portion may be formed, broadly, of one or more sheets, pads, or other layers of a thermal interface material. Such material may be formulated, for example, as a pressure-adhesive or other inherently tacky or otherwise self-adhesive composition which may rendered thermally conductive via its loading with one or more thermally-conductive particulate fillers such as aluminum oxide, magnesium oxide, zinc oxide, boron nitride, or aluminum nitride. The composition, which additionally may be formulated as a PCM, may be impregnated or otherwise supported in or on a reinforcement or other carrier such as a layer of a plastic or thermoplastic film, fiberglass or other fiber fabric, cloth, or web, metal foil, metal screen or other mesh such as, particularly, an expanded metal mesh. In the case of a metal foil or mesh, the carrier itself may render the composition, which in such instance may be filled or unfilled, thermally conductive in the case of an unfilled composition, or further thermally conductive in the case of a filled composition.
The body portion may be formed of a honeycomb-like material having a hexagonal or other open cellular structure which may be formed from a stack of sheets of metal foil, each of which sheets is corrugated along a longitudinal or lengthwise extent of the sheet to form, relative to a transverse or widthwise extent extending normal to the longitudinal extent, an alternating series of crests and troughs. The lengthwise extents of each of the crests of the sheets in the stack are bonded or otherwise joined, such as with an adhesive or solder, or by laser or spot welding, along the lengthwise extent of a corresponding trough of an adjacent sheet in the stack along a series of bondlines, with such bondlines between adjacent pairs of sheets being staggered. Laid-up and bonded as described, the stack forms an integral, lightweight, honeycomb-like cellular structure.
When employed in the heat sink of the present invention, such honeycomb structure provides a large surface area for convective or other dissipation of heat transferred through the base portion. By virtue of its light weight, such structure also results in a heat sink which resists shear and other forces which could cause it, in service such as in a laptop, cell phone, or other portable device, to detach from the electronic package to which it is bonded. Such structure, moreover, allows the network to exhibit a degree a flexibility or “spring” which allows the honeycomb to bend or otherwise conform with the base to accommodate curvatures and other deviations in planarity in the electronic package or other surface to which the base portion may be attached.
For optimum heat transfer efficiency, the structure of the honeycomb network may be oriented relative to the base portion such that the transverse extent of each of the sheets extends generally normal to the base portion. In this way, direct thermal pathways may be effected from the base along each of the individual foil sheets in the honeycomb, i.e., without the pathways having to cross the bondlines which otherwise would result in increased thermal impedance.
Advantages of the present invention include a lightweight, low cost, and efficient heat sink. Additional advantages include a heat sink which is conformable to electronic packages and other surfaces which may have curvatures or other deviations from planar. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.
The drawings will be described further in connection with the following Detailed Description of the Invention.